1. Field of the Invention
The present invention relates generally to electroplating and, more particularly, to an important method and apparatus for providing plating on the crests and valleys of an irregular target member.
2. Description of Related Art
A layer of metal is deposited on a manufactured article to enhance its aesthetics or increase its durability. Durability and low frictional characteristics are the primary concerns when the manufactured article will make frictional contact with other surfaces during use. An example of such an article is the spiral lobe rotor used in a downhole drilling motor that operates under the Moineau progressive cavity principal. The crests and valleys of the spiral lobe rotor make periodic sliding contact with the stator in a highly abrasive fluid. To prolong rotor life, it is necessary to plate the rotor with a material having low friction and good durability characteristics.
The prior art methods of electroplating irregular surface articles, such as the above-described spiral lobe rotor, have proved unsatisfactory in that the thickness of the plated material varies from crest to valley. In general, the crests are plated with more metal than the valleys. The prior art techniques sometimes yield a crest-to-valley thickness ratio as high as 8 to 1.
Premature rotor failure results from uneven plating because the relatively thin plating on the rotor valleys will wear away before the plating on the rotor crests. Moveover, increasing the plating time in order to provide adequate valley plating results in excessive crest plating, making it extremely difficult to accurately fit the rotor with the stator. Additionally, the stator cores around which the rubber elements of a Moineau system are injected can be better modified for better fit by using the controlled deposition method of the invention.